The present invention pertains to medication dispensing devices, and, in particular, to a portable medication dispensing device such as an injector pen.
Patients suffering from a number of different diseases frequently must inject themselves with medication. To allow a person to conveniently and accurately self-administer medicine, a variety of devices broadly known as injector pens or injection pens have been developed. Generally, these pens are equipped with a cartridge including a piston and containing a multi-dose quantity of liquid medication. A drive member, extending from within a base of the injector pen and operably connected with typically more rearward mechanisms of the pen that control drive member motion, is movable forward to advance the piston in the cartridge in such a manner to dispense the contained medication from an outlet at the opposite cartridge end, typically through a needle that penetrates a stopper at that opposite end. In disposable pens, after a pen has been utilized to exhaust the supply of medication within the cartridge, the entire pen is discarded by a user, who then begins using a new replacement pen. In reusable pens, after a pen has been utilized to exhaust the supply of medication within the cartridge, the pen is disassembled to allow replacement of the spent cartridge with a fresh cartridge, and then the pen is reassembled for its subsequent use.
One known type of injection pen uses a pull/push sequence applied to an externally accessible plunging member to deliver medication from the pen. The pulling of the plunging member in an axial, proximal direction first cocks or arms the pen for dose delivery, and then the axial pushing of the plunging member in the distal direction advances the cartridge piston to dispense the medication. While useful, such devices are not without their shortcomings. For example, some users who possess limited hand strength may find it difficult to apply sufficient force to push in the plunging member to inject a dose. In addition, the short plunging member travel associated with delivering very small doses in some pens may cause some users to question whether in fact a pen is operating to delivery the expected relatively small dose.
An injection pen disclosed in International Publication Number WO 96/26754 is designed with a mechanical advantage that may facilitate pen operation. The mechanical advantage is obtained with a gear set including first and second coaxial pinions that engage different racks within the pen, and which gear set travels with the pen thrust rod. While useful, due to the way the mechanical advantage is achieved, the mechanical advantage of the pen may be practically limited by how much smaller the diameter of the first pinion can be made than the diameter of the second pinion. In addition, the pen has a design which may be too complicated for some applications.
Another injection pen with a mechanical advantage is disclosed in International Publication Number WO 01/95959, which uses one or more gear wheels carried by a connector element threadedly engaged with the piston rod. The mechanical advantage of this pen may be practically limited by how small the gear wheel carried by the connector element can be made. Moreover, this pen has a relatively complicated design, as well as potentially costly components, such as separate springs.
Thus, it would be desirable to provide an apparatus that can overcome one or more of these and other shortcomings of the prior art.